1. Technical Field
The invention relates to a simulator for generating realistic simulations of acoustic signatures for sounds, particularly sonic booms, as perceived by a person on the ground.
2. Description of the Problem
The sonic boom generated by an aircraft flying at supersonic speeds has proven a major impediment in enhancing air travel. With the de minimus exception of the Concorde, and the briefly proposed Boeing “Sonic Cruiser”, essentially all increases in air speed in civil aviation occurred between 1903 and the introduction in the early 1960's of the Convair 990. For over forty years there has been little increase in the cruising speed of civil airliners and business aircraft.
The sonic boom undermined much of the economics of supersonic travel. Sonic booms proved damaging and annoying, which resulted in supersonic flight over land areas being banned. With supersonic flight limited to overwater flights, the market for such aircraft was both limited, and aircraft developed for such operations could not be economically redirected to overland use where their supersonic capabilities were of little use.
Eliminating the ill effects of the sonic boom would be facilitated by better understanding of the effects of sonic booms. Some of those effects are perceptual, that is, sonic booms are undesirable over populated areas simply because people do not like them. To better study perceptual issues a realistic sonic boom simulator is necessary.
The National Aeronautics and Space Administration has long recognized the need for sonic boom research and has built the Sonic Boom Simulator at the Langley Research Center in Hampton, Va. The Langley Sonic Boom Simulator is a person-rated, airtight, loudspeaker driven booth capable of accurately reproducing user-specified sonic boom waveforms at peak sound pressure levels up to approximately 138 dB. Input waveforms are distorted to compensate for nonuniformities in the frequency response characteristics of the booth and sound reproduction system.
The small size, and air tight features of the Langley facility underlie some of that facility's limitations. The facility takes a very direct approach in attempting to reproduce the sound of a sonic boom. The chamber is air tight and has rigid walls to support the low frequencies present in a sonic boom (i.e. to contain the slow pressure rise portion of the boom). However, most people experience a sonic boom under near free field conditions, which a small air tight chamber cannot replicate. NASA viewed sonic boom simulators as having inherently non-uniform frequency responses due to enclosure of the air space. The answer was to use complex computer algorithms to adjust components of the sound spectrum.